Silicon nitride components can be made by sintering silicon nitride powder. Such sintering can only occur in the presence of a liquid phase. Current formulations therefore include 10-20% by weight of one or more oxides including yttria, rare earths, alumina, magnesia etc. which, in combination with the oxide layer present on the surface of the silicon nitride, form a glass in which the latter can dissolve and reprecipitate to effect consolidation. Components made by this method are used as structural components intended to withstand temperatures as high as 1400.degree. C. and therefore have utilised those oxides such as yttria and the rare earths, which generate the most refractory glasses and crystalline silicates. This approach, together with the use of high purity silicon nitride powders and sophisticated control of microstructure during sintering, has resulted in the production of ceramics with flexural strengths of about 1 GPa at room temperature which retain a high proportion of this value at 1200.degree. and, sometimes, 1400.degree. C. However, this method incurs production costs which are sufficiently high to inhibit its use in high volume applications such as the substitution of metal components in the automotive and general engineering industries. These costs are due to high material costs, high sintering temperatures (often above 1800.degree. C.), and high process losses.
An alternative to starting with silicon nitride powder in the manufacture of silicon nitride components is to utilise metallic silicon powder which is available at a cost which is lower by about one order of magnitude. Nitridation of silicon powder at temperatures in the range 1150.degree.-1400.degree. C. is a well established process for the formation either of silicon nitride powder or of porous reaction-bonded silicon nitride components. Whilst the latter have a useful strength of about 200 MPa, which is retained to at least 1200.degree. C., together with excellent thermal shock characteristics, their porosity and poor resistance to wear make them unsuitable for use as engineering components.
Several authorities (including Mangels Ceram Eng Sci Proc 1981 2 589-603) have shown that silicon preforms containing sintering aids such as MgO, Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 can be nitrided at temperatures below 1400.degree. C. and sintered in nitrogen at temperatures in the range 1750.degree.-1850.degree. C. to form components having 98% of theoretical density. This sintered reaction-bonded silicon nitride has the advantage of a low overall linear shrinkage in the range 9-12% compared to a doubling of these values for components derived from a sintered silicon nitride powder. Despite this little commercial attention has been paid to the sintered reaction-bonded silicon nitride process because the reaction between silicon and nitrogen is highly exothermic and, if not conducted with care, easily results in localised heating of preforms to temperatures exceeding the melting point of silicon (1420.degree. C.). This problem increases with the size of the furnace; nitridation times of about one week are typical. Also, after this slow nitridation stage, sintering is effected in a second graphite furnace with the component embedded in a protective powder bed. Furthermore, the mechanical properties of sintered reaction-bonded silicon nitride components are generally slightly inferior to and more difficult to control than those from the more direct sintered silicon nitride route.
Pompe (U.S. Pat. No. 4,492,665) has shown that the addition of 15-50% of fine silicon nitride powder to silicon powder acts as a dispersing aid and allows the latter to be ground using a ball mill to a grain size of &lt;1 .mu.m. The addition of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 powders as sintering aids further dilutes the silicon and effectively mitigates the effects of the exothermic reaction during nitridation, at least on a small scale. Sintering was effected in a separate graphite furnace with the component embedded in a layer of protective powder containing Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and AlN with a temperature hold at 1770.degree. C. and a peak temperature of 1850.degree. C. Pompe also suggested that the nitridation and sintering steps could be carried out in a single furnace, although no supporting examples were provided.
More recently, Tiegs and co-workers (Ceram Eng Sci Proc 14 1-2! 378-388, 1993) used microwave heating to produce sintered reaction-bonded silicon nitride using a one stage nitriding/sintering process with the component packed in a powder bed. As yet, this process is confined to a laboratory scale.
An obvious alternative to the use of yttria or rare earths as sintering aids is magnesia and this has been used both as such and, in conjunction with alumina, in the form of spinel MgO.Al.sub.2 O.sub.3. A disadvantage however lies in the tendency of MgO to react in a carbonaceous environment according to: EQU MgO+C.fwdarw.Mg+CO
to give products which are gaseous at temperatures &gt;1700.degree. C. Considerations of their volatility also exclude many other species such as the oxides of the alkali metals and boron.
Calcium oxide, CaO, has not been proposed as a sintering aid for the manufacture of sintered reaction-bonded silicon nitride because the presence of calcium has been demonstrated to lead to a deterioration in the high temperature mechanical properties of the ceramic. Calcium has recently been found to be one of the cations which can promote the formation of .alpha.' sialons Ca.sub.x Si.sub.12-m Al.sub.m O.sub.n N.sub.16-n by heating compacts of appropriate quantities of CaO, .alpha.Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and AlN to temperatures in the range 1750.degree.-1800.degree. C. Although superficially attractive, this process is difficult to control whilst aluminium nitride, AlN is both relatively expensive and can introduce processing problems because of its susceptibility to hydrolysis.
Against this background, there is a requirement to provide a method of manufacturing sintered silicon nitride components suitable for high volume applications at temperatures below 1000.degree. C. using a fast, energy efficient process starting from low cost commodity raw materials.
The invention provides a method of manufacturing a dense sintered reaction-bonded silicon nitride component, the method comprising forming a powder mixture comprising substantially pure silicon and oxide or oxide precursor additives, forming a preform from said powder mixture, the preform being generally in the shape of the component, reacting silicon in the preform with nitrogen to form silicon nitride, and sintering said silicon nitride, characterised in that said additives comprise alumina and a calcium compound.
By "substantially pure silicon", we mean unreacted silicon which will, however, inevitably, have a thin film of oxide thereon.
A method according to the invention utilises inexpensive materials and can be used to make components which have over 95% of theoretical density (typically 97 to 99%) which have good mechanical properties and can withstand temperatures up to 100.degree. C. Thus, the components are cheap enough for use as, for example, automotive components, and are suitable for use in many applications. The use of a calcium compound enables the sintering to take place in the presence of calcium oxide which is found to allow sintering to high density at relatively low temperatures. The components produced by this method were found to have a surprisingly good surface finish with no evidence of pitting or discolouration.
The additives may form 5 to 15% by weight of the powder mixture, for example there may be about 5%, about 6% about 7.5%, or about 9.5% by weight of said additives depending on the particular additives. The alumina may form 3 to 10% by weight of the powder mixture.
Said additives may also comprise magnesia which is found to lower the peak sintering temperature necessary and thereby reduces the cost of the method. Magnesia may form up to 3.0% by weight of the powder mixture.
The calcium compound may be calcium oxide which may form 1 to 3.5% by weight of the powder mixture. Where only calcium oxide and alumina were present in the additives, it was found to be desirable to maintain a molar ration of 1:1 to 1:1.5. Where magnesia was also present, a molar ratio of 0.5:0.5:1 was found to be suitable (alumina=1). As an alternative to calcium oxide, a precursor thereof may be used in sufficient quantity to provide said quantity of calcium oxide. Thus, the calcium compound may be calcium carbonate which reacts to provide calcium oxide, with carbon dioxide being driven off in the processing.
As mentioned above, the reaction of nitrogen with silicon is highly exothermic, and it is important, therefore, to control this reaction to ensure that the silicon does not melt. On economic grounds, however, it is desirable that this reaction takes place as rapidly as possible so that time in the furnace is minimised. These requirements can be achieved, in a method in accordance with the invention, by using a nitriding process comprising heating the preform in a furnace under a nitrogen atmosphere while controlling the temperature in the furnace and allowing nitrogen to flow into the furnace on demand, the flow rate of nitrogen into the furnace being monitored, wherein the nitriding process comprises the following phases:
a pre-reaction phase during which the temperature is raised to cause the reaction to commence; PA1 a first reaction phase during which the temperature in the furnace is held at a substantially constant level and the flow rate of nitrogen is controlled so that it does not exceed a predetermined maximum rate, the first reaction phase being initiated when said flow rate of nitrogen exceeds a first predetermined level, and being terminated when said flow rate falls below a second predetermined level; and PA1 a second reaction phase during which the flow rate of nitrogen is maintained between predetermined upper and lower limits by controlling the temperature in the furnace.
This nitriding process also allows the heat generated in the reaction to be utilised so that the method is energy-efficient.
During the said pre-reaction phase, the temperature may be raised at 80.degree. to 120.degree. C. per hour, e.g. at about 100.degree. C. per hour, until a temperature of 950.degree. C. to 1050.degree. C., e.g. about 1000.degree. C., is achieved and at a slower rate thereafter, e.g. at about 40.degree. C. per hour, until nitridation of the silicon commences. The temperature may then be further raised at a slower rate, e.g. at between 5.degree. C. and 20.degree. C. per hour, until said flow rate of nitrogen exceeds said first predetermined level.
Because the amount of nitrogen required to react with the silicon depends on the quantity of silicon present, said predetermined maximum flow rate of nitrogen is, preferably, determined as a function of the total weight of silicon in the furnace, a linear function being suitable. For the same reason, said predetermined upper and lower limits on the flow rate of nitrogen, applied in the second reaction phase, may also be determined as a function of the total weight of silicon in the furnace.
The second reaction phase of the nitriding process may be terminated when the temperature in the furnace reaches a predetermined level. This may terminate the nitriding process or, in order to ensure more complete reaction of the silicon with nitrogen, after the second reaction phase, the method may comprise a third reaction phase during which the temperature in the furnace is maintained substantially constant and the flow rate of nitrogen is allowed to drop.
It is preferable that, prior to said nitriding process, said preform is heated under vacuum to remove water vapour and any other volatile materials present. The preform can be heated, preferably in the same furnace used for the nitriding process, to about 800.degree. C. and held at that temperature for about 1 hour under a vacuum of about 0.01 Torr. The nitriding process can then be commenced without allowing the preform to cool.
In order to minimise both energy use and time required, it is preferable that the silicon is reacted with nitrogen and the silicon nitride so formed is sintered in the same furnace in a continuous operation. The sintering involves the application of heat and gas pressure.
The temperature reached in sintering may be about 1725.degree. C., and the pressure reached may be between 5 and 11 bar (500 to 1100 kN/sq. meter), e.g. about 10 bar (1000 kN/sq. meter).
The invention also provides a sintered reaction-bonded silicon nitride component made by a method according to the invention. The component may be in the form of a valve train component for an internal combustion engine, e.g. a tappet shim, a roller follower, or a rocker insert.
There now follows a detailed description, to be read with reference to the accompanying drawings, of an illustrative method of manufacturing a sintered reaction-bonded silicon nitride component in accordance with the invention.